Soil's Role in Carbon Sequestration

The Dirt on Climate Change
Could soil engineered specifically to maximize carbon storage dampen some effects of climate change? Very possibly.
By Peter Friederici
Miller-McCune, December 21, 2009
Straight to the Source

"Imagine all the organic stuff that comes into a city — and then imagine putting all that carbon into the soil," said William Woods, University of Kansas geographer.

Conflicts tend to scatter people, and ideas, in unexpected ways. After the American Civil War, a flood of so-called Confederados fled the devastated South and set up farms in the Brazilian Amazon. They planted rice and sugar cane and tobacco, and they prospered. But the lands they settled - primarily high bluffs along rivers - weren't any more pristine than Alabama or the Carolinas had been. As they plowed, the settlers unearthed vast quantities of potsherds that showed the land had been inhabited before. And the ceramics weren't the only sign of previous human cultivation: The deep black earth itself, very different from the pale, nutrient-poor soils of much of the Amazon, quickly revealed that people had been indispensable in creating its fertility.

"The rich terra preta, 'black land,'" of one settlement was "the best on the Amazon. ... a fine, dark loam, a foot, and often two feet thick," wrote an American naturalist named Herbert Smith in 1879. "Strewn over it everywhere we find fragments of Indian pottery. ... The bluff-land owes its richness to the refuse of a thousand kitchens for maybe a thousand years."

Though they have always been prized by farmers, the dark soils of the Amazon were largely forgotten by science for a century after their discovery. They are now re-emerging as an important topic of study, not because they're an ethnographic or historical curiosity, but because they show an exceptional ability to store carbon, which in the form of carbon dioxide has rapidly turned into one of humanity's most pernicious waste products. As a result, they're joining the rapidly growing roster of tactics that might be used to combat climate change. Researchers around the world are considering whether people may, by engineering soils specifically to maximize carbon storage, be able to absorb substantial amounts of our emissions, increase the fertility of agricultural areas and dampen some of the effects of climate change.

Sound utopian? Maybe. But as the long aftermath of the Civil War shows, solutions to deeply ingrained social problems often do emerge - though not always quickly and certainly not without enormous and sustained effort.

"We could gear up for this with something like the Manhattan Project," says William Woods, a University of Kansas geographer and expert on terra preta. "Imagine all the organic stuff that comes into a city - and then imagine putting all that carbon into the soil. It works, though we aren't there yet. So far no one seems to have the will do it."

Carbon is the essential building block of all life, the bustling captain of industry, the stuff at the core of diamonds. Carbon has long starred quietly in virtually everything that goes on in human lives, but now its blandly essential air has been eclipsed by a new role: that of villain in the long-running drama of climate change. As the key component of carbon dioxide, element 12 has now firmly moved in the public mindset from good guy to a problem that threatens the future of the very lives it has made possible.

Carbon dioxide isn't the only greenhouse gas out there - methane, the nitrogen trifluoride used in the manufacture of flat-panel televisions, and others contribute to global climate change, too - but it is the most widespread and the one most directly associated with the industrial revolution. Combustion begets CO2, simply, and as that extra gas accumulates in the atmosphere, it causes the Earth to retain more heat. The litany of effects that result from that warming is becoming increasingly well known: rising oceans, more severe heat waves, irregular precipitation, greater threat of drought. So is the precise concentration of carbon dioxide in the atmosphere, which has been rising steadily since humans started burning a lot of coal in the 19th century - and which is currently rising at a rate faster than anticipated by most of the predictions made by the Intergovernmental Panel on Climate Change.

Carbon helps form the organic molecules that comprise pansies and panthers, redwood trees and blue whales. When these organisms die, the carbon in them eventually returns to the environment, often by oxidation as carbon dioxide. How much carbon a given ecosystem stores, then, is a matter of dynamic flux that can be measured on a variety of different time scales. Some ecosystems can store carbon effectively enough that scientists refer to them as "carbon sinks" — that is, they hang onto carbon for decades or centuries, long enough that they contribute to lowering atmospheric concentrations of CO₂ and perhaps reduce the impacts of climate change. Grow a forest, and it accumulates carbon slowly, perhaps for centuries. Burn it down in a severe fire, and most of its carbon goes up in smoke. Cut it down for lumber and the carbon in that wood may lie undisturbed for centuries, while that in the leaves, unharvested branches and disturbed soil is quickly released into the atmosphere. Other ecosystems follow the same pattern but so much more quickly that no one refers to them as carbon sinks: In June, an Iowa cornfield rapidly sequesters carbon as the crop plants grow; in November, it releases the element as the chopped stalks degrade.

But it's not just plants and animals that hold carbon. Soils do, too, a lot of it — an estimated 2.5 trillion tons worldwide, or more than three times the amount floating around in the atmosphere and about four times as much as in all the world's living plants. About 60 percent of the soil's carbon is in the form of the organic molecules that compose living things, while the other 40 percent is in inorganic forms such as calcium carbonate, the crusty salt common in desert soils. Unfortunately, people have not been very kind to the soil's pool of organic carbon, at least not since the dawn of agriculture. According to the IPCC, human beings were responsible for the emission of about 270 billion tons of carbon from the burning of fossil fuels between 1850 and 1998. During the same period, they caused the loss of about half that much carbon from terrestrial ecosystems through such activities as logging and plowing; all told, disturbances to soils during that century and a half caused the emission of about 78 billion tons of carbon. In other words, though the burning of fossil fuels is the main culprit in climate change, our land uses have played an important supporting role.

"If you convert from prairie or forest to agriculture, the soil's organic carbon decreases very rapidly," says Rattan Lal, the director of the Carbon Management and Sequestration Center at Ohio State University. "It can decrease by as much as 30 to 50 percent in a relatively short time. Most soils in Ohio have lost between 10 and 40 tons per acre of carbon because of blowing, drainage, erosion, removal of crops for feeding cattle, removal for biofuels and other factors. The carbon storage capacity of these soils is like a cup that's now only half full."

To soil scientists such as Lal, humanity's recent history with dirt constitutes a triple whammy. All the carbon that's been removed from soils has helped to push up carbon concentrations elsewhere in the biosphere, whether in water, where it contributes to the acidification of the oceans, or in the air, where it contributes to the baleful effects of climate change. As soils have lost carbon, they also have lost a good deal of their productivity. They store less water, harbor fewer microorganisms, are less able to transfer nutrients to plant roots, require more fertilizer. In their impoverished form, they're also less able to store carbon than they once were. They've gone from sink, in many cases, to source.

That's a big problem, Lal says, but he is one to see soil's cup as half full, rather than as half empty: Saving the planet's soils, he says, may also mitigate at least some of the impacts of climate change. And it's vital, too, for the most visceral of reasons.

"We have 6.7 billion people now," he says. "We'll have 10 billion in a few more decades. How are we going to feed them if we don't take care of our soils?"

Plants have countless benefits, but to climatologists they're basically pumps that channel carbon from the atmosphere as they photosynthesize. They use much of it in constructing their own lasting tissues, but they also transmit a lot of it as they absorb nutrients from soil. According to David Manning, a soil scientist at the University of Newcastle, plants move about as much carbon underground as they do into wood and leaves.

"When we normally think about fixing carbon by plants, we think about forests," he says. "But when you see the carbon stored in a forest, you have to think that there's as much underground as there is aboveground. It comes out through the roots as a complex cocktail of compounds, such as citric acid, that break down the nutrients in the soil."

This function of plants happens to connect the organic and inorganic roles of carbon. Most of the carbon in soils is in organic material — it's the rich brown stuff that makes a vegetable garden thrive. But many soils also contain a lot of carbon in highly stable, inorganic forms such as calcium carbonate. That's well known to farmers and ranchers in the western United States and other arid regions, where a hard white crust known as caliche often forms on or within soil. These carbonates form readily where insufficient rain falls to wash them away, but Manning has found that they also form, often at greater depths, even in climates as wet as Britain's. All that's needed is a source of calcium, and the right plants to emit carbon through their roots.

As it happens, people have inadvertently been putting calcium into British soils for hundreds of years. When buildings are demolished and their bricks, mortar or concrete debris discarded, calcium is freed up. Manning's research team has found that urban sites in that country can sequester as much as 10 tons of carbon per acre each year, not by the creation of organic material but rather by the formation of long-lasting carbonates.

"It's fascinating," he says. "We bring up old house bricks, and they're covered with lumps of calcium carbonate. Typically we find that the urban soils we look at contain up to about 20 percent calcium carbonate."

Though this process takes place on its own, Manning thinks that careful planning could help speed it up. For example, choosing the right sorts of plants for urban landscaping could maximize the production of carbonates. He notes, though, that this sort of carbon sequestration in urban soils is a zero-sum game. The manufacture of cement produces huge amounts of carbon dioxide, and waste construction or demolition debris in soil can never bind to more carbon than has been produced in its manufacture.

"The scale of production of cement is so great that you could never do more than compensate for the production process," he says. "But this can help close the loop. It may help get rid of the word 'waste,' which is a horrible word. And if carbon trading really takes off, then to be able to demonstrate that the carbon on your site has ended up as carbonate might have a value."

In theory, people may be able to remove large amounts of carbon from the atmosphere by taking advantage of the caliche formation that goes on naturally in the world's vast arid areas. Calcium is readily available in natural form in seawater, so why not simply put a lot of it on desert soils to form lots of carbonate and remove CO₂ from the atmosphere?

"We could probably sequester vast amounts of carbon by adding calcium to desert soils," notes Curtis Monger, a soil scientist at New Mexico State University who studies carbonate formation. "But at what point do we become concerned about turning our desert soils to stone? Whenever we talk about global-scale geoengineering, we don't mean to, but we tend to mess things up."

It's difficult to discuss the modification of desert soils as a carbon-sequestration strategy in much detail because these soils are little understood at this point. Several teams of desert researchers, including Monger's, have been surprised in recent years to find that tracts of arid land seem capable of absorbing far more carbon dioxide than can be explained according to standard models of how these ecosystems work. He remembers one experiment in which his team was measuring CO₂ being emitted from soil, only to find that the gas was suddenly sucked back down into the earth.

"We wondered whether our instruments were screwy," Monger says. He thinks that light precipitation may have caused a sudden surge in carbonate formation, removing the gas. But he notes that the study of desert soils, especially of their link to the global carbon cycle, is in its infancy.

"It's the quiet before the storm," he says. "The IPCC still hasn't recognized desert soils and calcium carbonate as a big player. But it will."

If the soils of desert areas are a wild card in the high-stakes game of climate change, biochar is increasingly coming to look like a royal flush — a reliable winner. The idea behind it is very simple: To get rid of unwanted carbon, put it directly into the soil. Farmers do this all the time, of course, when they till the harvested parts of crop plants back into a field — but under typical agricultural conditions some 90 percent of the carbon in these residues quickly winds up back in the atmosphere. The idea behind biochar, instead, is to convert that carbon before plowing it under by first turning it into durable charcoal.

That's exactly what the native peoples of the Amazon were doing for many centuries before Spanish and Portuguese explorers arrived. According to geographer Woods, the large-scale use of biochar in South America probably arose some two-and-a-half millennia ago, at about the time that corn was becoming a widespread food crop. This ready source of food led to increased human populations, centralized villages and pressure to increase yields. It could not have taken long before farmers observed where the lastingly fertile soils were: namely, in the places where charcoal and organic wastes were discarded.

"They're seeing that this stuff is fertile; they're putting their gardens there; and it's not a big step from there to creating it deliberately," Woods says. "The carbon in the form of charcoal is an integral part of these soils, and it happens to take a great deal of carbon out of the atmosphere."

Those farmers didn't need to worry about climate change, but they were taking advantage of a fundamental property of carbon in the form of charcoal: It has a complicated structure, and it lasts a long time. That's why charcoal does such a good job as a filter. Its complex structure provides many places where other molecules can linger, whether they're impurities in whiskey or nutrients that plants need. As a result, soil fertility can increase a great deal when charcoal is combined with organic materials that provide nutrients. Those terra preta soils in the Amazon don't just contain much more organic material than other soils; they also hold onto potassium, phosphorus and numerous trace minerals much more readily and provide much better microhabitats for such important organisms as bacteria and fungi. And because charcoal takes so long to break down, terra preta soils retain their fertility much longer than those of other tropical areas.

Robert Brown began thinking about biochar as a side effect of working on gasification, which is a means of converting organic materials into energy with great efficiency by first turning them into a gas, then burning them. Brown, an engineer at Iowa State University, was struck by how difficult it is to burn the last small bits of charcoal even in the hottest and cleanest of fires. Fine, he thought — the charcoal, after all, is a carbon sink, and because it's itself a filter, it is not a pollutant.

"My notion was we had to put it in old coal mines to get rid of it," he says. "But in fact it's so recalcitrant that you can just bury it in soil to get rid of it."

Brown and colleagues are currently working on a small pilot plant that will convert unneeded organic material from Iowa cornfields into ethanol and charcoal. The idea is that farmers wouldn't harvest only ripe ears of corn come fall; they'd also harvest about half of the remaining plant fiber — which farmers call stover. Then they'd drive the stover to a nearby plant, where gasification and a reaction with a catalyst would turn the biomass into ethanol and some fine particles of leftover charcoal — about 300 pounds of it for every ton of stover. The latter could then be applied to fields, where it would both enhance soil fertility and act as a carbon sink. The corn stalk-based ethanol, meanwhile, wouldn't compete with food production in the way that ethanol produced from corn kernels does.

If charcoal would increase the health of Iowa's soils, Brown says, think of how much more it would help generally nutrient-poor tropical soils: "I think this could be a revolution for agriculture. It could dramatically increase the efficiency especially of tropical agriculture. If you were to establish a farm and sequester carbon there, you'd not only produce crops but improve the soil, too. So you wouldn't have to burn down another tract of forest a few years down the road."

Still, there a lot of kinks to be worked out before what manifestly works in the lab can be put into action in an Iowa cornfield, or in the Brazilian jungle. A number of researchers and entrepreneurs are trying to resolve some of those issues, by designing and testing the gasification burners that would be required, or calculating what other nutrients would need to be applied along with biochar to maximize soil productivity. But it's likely that some of the thorniest issues will play out on the ground. Some observers worry that biochar will become such a promising means of combating climate change that its production will trump other values; they envision nightmare scenarios in which huge tracts of forest are axed only for the value of the charcoal they can produce. As Monger points out, large-scale geoengineering always seems to bring out a new set of problems.

"You have to think about it from a sustainability perspective," says Johannes Lehmann, a leading biochar expert at Cornell University. "It makes no sense to use pristine rainforest for biochar production, or to produce biochar in Iowa and ship it to West Africa. Biochar should not be seen as an alternative to best management practices, but in addition to them."

If biochar is beginning to seem like a sort of silver bullet that would allow us to shoot our way out of our climate quandary, then it's time to take a deep breath. It's not. Though many questions about it remain to be answered, its use may indeed prove a relatively inexpensive way to improve soil fertility, to find a productive use for many products — especially agricultural leftovers — that are currently considered waste and to sequester some carbon. But the harsh reality of the carbon cycle, and of climate change, is that there is no single solution that can get humanity out of its self-inflicted crisis.

A number of scientists have tried to estimate how much carbon people may be able to pump out of the atmosphere through the application of biochar. In a recent paper, James Hansen, the NASA scientist who has been a prominent voice on climate change for many years, and colleagues estimated that large-scale adoption of biochar sequestration could reduce atmospheric CO₂ by about 8 parts per million by 2050. Ohio State's Rattan Lal claims that widespread use of biochar, in conjunction with other wise agricultural stewardship such as erosion control and no-till farming, could sequester some 1.25 trillion tons of carbon a year. By itself, that could cause atmospheric CO₂ levels to drop about 50 ppm over the next century.

That's a lot, but still far from enough, given that the current level of CO₂ is 387 ppm — up from about 315 in the late 1950s — and rising at the rate of about 2 ppm per year. Climatologists point out that the global carbon cycle appears to be experiencing some feedback loops through which warming begets more warming. As ice in the Arctic and on mountain glaciers melts, the newly exposed water or land surface is darker and absorbs more energy from the sun. As Arctic tundra warms, frozen peat decomposes, releasing both carbon and methane — itself a potent greenhouse gas. As once-lush forests dry out, they're more subject to large-scale fires that release enormous amounts of carbon dioxide. As the oceans warm, they become less able to absorb CO₂ from the atmosphere. And so on — the list is dispiriting.

Hansen and a number of his colleagues have called for a target CO₂ concentration of 350 ppm to avoid some of the worst effects of runaway climate change. As the human population and its energy demands both grow, there will be no way to get there without a widespread embrace of numerous conservation and sequestration tactics. It's politically tricky to both reduce emissions and increase carbon sequestration at the same time; embracing a solution with the potential to store lots of carbon may reduce the imperative to reduce carbon emissions in the first place. As Lehmann told the U.S. House Select Committee for Energy Independence and Global Warming in June, "Biochar must not be an alternative to making dramatic reductions in greenhouse gas emissions immediately, but it may be an important tool in our arsenal for combating dangerous climate change."

About a week after Lehmann testified, the House passed a climate bill that includes a cap-and-trade system giving polluters incentives to pay to offset their carbon emissions. Though many environmentalists criticized the bill as far too little, far too late, it at least opens the door to valuing projects that sequester carbon as an offset to emissions and dovetails nicely with the potential for finding money to pay for the widespread application of biochar.

It may be, then, that future farmers — much like those of the ancient Amazon — will ultimately be judged not only on what they can extract from the soil but also on what they put in. To biochar advocates such as Rattan Lal, that's a step in the direction of good stewardship — and good economics.

"Let us pay farmers for ecosystem services," he says. "If they improve the quality of their soils, if it's good for erosion control, for biodiversity, for climate mitigation — let us pay them for those services."